1) Field of the Invention
A safety device for disabling a positive pressure radiant tube heater upon failure of the heat exchanger and a corresponding method are described. In accordance with the current revision of ANSI Z83.20 (the standard for low intensity radiant heaters), radiant tube heaters are mounted within an enclosed space such that the top, two sides and the bottom of the heaters are simultaneously arranged such that the maximum surface temperature on any combustible item, such as a wall, does not exceed 90 degrees above ambient, to determine the clearances to combustible materials distances. This invention relates to the ability to detect a condition where a failure of the heat exchanger (which may result in elevated temperatures within the clearance to combustible distance above the heater that can cause a potential fire hazard) triggers the safety device and disables the heater. The safety device includes a low melt wire, preferably an insulating sleeve positioned about the low melt wire, the wire and sleeve positioned on top of the reflector, a tension device to maintain the wire under tension, whereupon if the wire is or becomes discontinuous the heater will be disabled.
2) Prior Art
Radiant tube heaters are commonly known to comprise the following: a burner assembly incorporating a gas valve, ignition control, pressure switch, combustion air blower, burner head, a heat exchanger emitter tube functionally designed to enclose the flame and products of combustion during operation of the heater, a reflector to downwardly direct the heat from said heat exchanger, and a suspension system to support the weight of said burner assembly, heat exchanger tube, and reflector.
There are currently two distinct methods of combustion air inducement for radiant tube heaters. The pull system has a burner which is at the opposite end of an exhaust end, and the air for combustion is induced into the system under negative pressure. Generally, a blower is mounted on the exhaust end to create the negative pressure. In the push system, the air is supplied into the burner end of the tube heater under positive pressure and the heat exchanger is pressurized.
Principally radiant tube heaters are designed to operate at a plurality of different heat input rates, from about 25,000 Btu/hr up to about 300,000 Btu/hr, where the common adjustments for input rate include: adjustment of gas pressure, change of gas injector size, air metering plate, combustion air blower, and the total length of heat exchanger. Substantially, radiant tube heaters are designed with the intention of making the flame as long as possible for each input rate in order to spread the heat as far along the heat exchanger as possible. This design intention results in a different location of the highest temperature measured on the surface of the heat exchanger tube. This can vary between about 12 inches to about 96 inches along the length of the heat exchanger. The highest temperature location will vary for the same heat input depending on the length and configuration of the heat exchanger tubes.
A potential cause for increased temperature above the reflector, which would result in excessive temperatures within the distance specified as the clearance to combustible materials, is if the heat exchanger fails and the products of combustion are under positive pressure. Specifically, if the heat exchanger of the tube heater has a burn through, for example, then flames and products of combustion, under pressure, will exit the heat exchanger. Combustible material that was a safe distance away from the heat exchanger is now in jeopardy of burning. Moreover, the preferred material for the reflector is aluminum for its reflective properties. Under these conditions the reflector will melt allowing the flame to penetrate through the reflector.
For this reason the current state of the art is to use higher service temperature heat exchangers, typically Alumatherm® metal is a commonly used material in the manufacture of the heat exchanger tube. The premise is that with the higher service temperature materials, there usually is sufficient safety factor to prevent heat exchanger burn through, and reduce the risk of elevated temperatures within the clearance to combustible distance above the heater that can cause a potential fire hazard.
This invention is concerned with providing an extra safety device such that when an unforeseen tube failure has occurred, the heater will turn off safely.
Heat sensing devices exist as commercially available materials, such as heat sensitive wire, wire that will make contact due to high temperature, bimetallic switches, fusible links, and thermal fuses. Normally because the exact location of the heat exchanger failure is difficult to predict and it will vary depending on the heat input rate, these devices are expensive because they must be positioned every couple of inches from one another to insure a burn through of the heat exchanger is detected. Thus a need exists for a safety device that continuously covers a length of the heat exchanger where it is predicted a failure can occur, typically the first section of heat exchanger (usually within the first 10 feet).